(a) Field of the Invention
The present invention relates to a photosensitive material for electrophotography and, in particular relates to an amorphous silicon system photosensitive material for electrophotography wherein a photoconductive layer comprises a lamination of three layers, each of which has a specified function.
(b) Description of the Prior Art
The photoconductive materials for the electrophotographic element usually have been inorganic materials such as Se, ZnO, CdS and the like and organic materials such as poly-N-vinylcarbazole, trinitrofluorenone and the like, and amorphous silicon(a-Si) has recently come to attract public attention. Amorphous silicon not only is possessed of characteristics being equal to the conventional Se photosensitive material but is superior in respect to safety, durability and the like against the environment.
The electrophotographic photosensitive materials have been demanded to satisfy collective characteristics such as chargeability in the dark, sensitivity at the time of light radiation, residual potential at the time of repetitive use and the like, and further have been demanded from various respects to have the physcial properties inherent in the photoconductive materials per se to decide these electrophotographic characteristics.
However, it is difficult for a single photoconductive material to satisfy every requirement completely. Therefore, it has been tried to improve the electrophotographic characteristics by providing a layer structure of a photosensitive layer using plural photoconductive materials, combining a photosensitive layer with a high resistance layer or the like.
In addition to the principal organic photoconductive materials, there have been proposed various other photoconductive materials, for instance, those used mainly as a functional partition type photosensitive layer that comprise the combination of a charge carrier generating layer that generates charge carriers on the radiation of light, a charge transfer layer that transfers said charge carriers and those used for each of these layers.
Further, since it is demanded that a photosensitive layer as a whole to have a high dark resistance, it has been proposed to make an amorphous silicon system electrophotographic photosensitive element highly resistive. As the methods therefor, there have been proposed:
(a) a method for making a photosensitive layer as a whole have a high resistance of 10.sup.14 .OMEGA.cm or more by providing blocking layers on both sides of said photosensitive layer; and PA0 (b) a method for attaining a high resistance of 10.sup.13 -10.sup.14 .OMEGA.cm or more by incorporating oxygen, carbon and nitrogen atoms in an a-Si layer and rendering said layer photoconductive.
In the above method (a), as shown in FIG. 11, the charged potential, constituting one important factor of the electrophotographic characteristics, is held by upper and lower blocking layers 45, 47, and and a-Si layer (photosensitive layer) 43 free from oxygen, carbon and nitrogen atoms is used as a charge carrier generating layer that generates charge carriers on incidence of light. In this figure, the reference numeral 41 stands for a substrate. As the materials for of this blocking layer, there are used high resistance materials such as metal oxides, organic matters and the like in addition to a-Si containing oxygen, carbon and nitrogen atoms. These blocking layer materials display either an electrical insulating property or a non-photoconductive property (which see Japanese Laid-open Patent Application No. 52178/1982 Specification).
In this instance, it has also been considered to form the blocking layer by endowing the a-Si layer (charge carrier generating layer) and the blocking layer materials with counter semiconductivity (for instance P-N junction, N-P junction and the like) in relation to those layers, and to use various metal materials in the substrate for forming a Schottky barrier between it and the layer thereon. According to the method (a) mentioned above, however, there is no way for improving the characteristics of the blocking layer in order to control the charged potential of the electrophotographic photosensitive element, and accordingly process control of the electrophotographic characteristics becomes difficult because the way of obtaining a desired electrophotographic element by changing the film thickness of the photosensitive layer can not be employed as usual. When a high resistant material is used in the blocking layer, there are brought about defective electric characteristics such as rise in residual potential, fall in repetition characteristics and the like because this material is non-photoconductive or electrically insulating, and so the carriers generated by the photosensitive layer are trapped within or by the interface of the blocking layer.
On the other hand, in the above method (b), as shown in FIG. 12, a photosensitive layer 43 comprising adding oxygen, nitrogen and carbon atoms to an a-Si layer is formed on a substrate 41 in order to hold the charged potential. And, both a blocking layer 45 and a protective layer 49 are also allotted a part in improving the chargeability (which see Japanese Laid-open Patent Application No. 115553/1982 Specification). According to this method, control of the charged potential can be effected readily by changing the film thickness of the a-Si layer 43, but it is difficult to improve both the photosensitivity and the chargeability simultaneously. Therefore, more improvement has been demanded.